The present disclosure relates to a method and apparatus for attenuating RF signals and to a bidirectional communications system incorporating such an apparatus. In one embodiment, the disclosed apparatus can be used to control return path attenuation in a bidirectional cable (“CATV”) system.
Many communication systems include a head end and a plurality of subscriber units. A forward path carries signals from the head end to a subscriber unit and a reverse path carries signals from the subscriber to the head end. In a frequency division duplexed communication system, forward path and reverse path signals may be transmitted in separate and non-overlapping frequency bands.
One example of such a system is a bidirectional CATV system with a head end controlled by a system operator and with a plurality of subscribers' televisions equipped with set top boxes or cable modems. Signals on the forward path of the CATV system include broadcast television channels; signals on the reverse path include data related to video on demand, internet access, or other services provided by the system operator. In one possible configuration, the CATV system is hybrid fiber optic/coaxial with at least one node between the head end and the subscriber. In such a system, the signals are transmitted between the head end and the node via optical fiber, and between the node and the subscriber unit via coaxial cable.
Performance of communication systems such as a CATV system can be significantly improved by appropriate power control, including appropriate attenuation of signals. This attenuation can reduce ingress noise and equalize different signal levels from cable modems at different taps to improve the signal to noise ratio. Forward path power in a CATV system is controlled more easily than return path power because all forward path signals originate from one location. Loss in the return path may be more difficult to manage because of the great variance from tap to tap. For example, the distance from the subscriber to the node affects the attenuation the return path signals will experience in the coaxial cable. In another example, ingress noise can result from RF sources near open taps in or near the subscriber's location. Moreover, conditions may be time varying, as where an interfering RF source is intermittent or varies in power with time.
For maximum performance, return path power should be tailored to the specific properties of each subscriber. A prior art device that provides attenuation in a CATV system is a simple high-pass, low-pass, or band-pass filter. A filter can attenuate signals in one frequency band but pass signals in a different frequency band without attenuation. Such a filter is disadvantageous, however, because outside of a transition range it provides either very large attenuation or no attenuation at all to a signal at a particular frequency. Thus a filter either does not attenuate the return path or it inhibits all communications on the return path. Better performance will be achieved if attenuation levels can be selected precisely at each tap.
Another prior art device that provides attenuation in a CATV system is a passive step attenuator. A passive step attenuator can be designed to provide flat loss in a particular frequency band. Thus a system operator can in theory design a passive step attenuator with the appropriate attenuation for each subscriber.
Reliance on the passive step attenuator, however, has a number of disadvantages. First, the passive step attenuator must be manufactured and sold separately from the cable modem itself because the needed attenuation cannot be determined until the cable modem is installed at the subscriber's location.
Second, the producer must make multiple versions of the passive step attenuator, each providing a different level of attenuation for a particular frequency band. This imposes additional expense on the system operator and may not be able to ensure optimal performance of the system. The system operator must install at each subscriber's location the passive step attenuator or combination of passive step attenuators that most closely provides the level of attenuation it deems appropriate attenuation for that subscriber. Although the system operator can achieve greater resolution in attenuation as the producer makes more versions of the passive step attenuator, it may never be able to provide the precise attenuation deemed appropriate for each subscriber.
Finally, a passive step attenuator cannot adjust to time-varying conditions. Proper attenuation of signals in a CATV system requires that stronger RF signals be attenuated to a greater degree than weaker RF signals. RF signals may vary with time due to changing channel conditions. Ingress noise, for example, may temporarily increase because of the emergence of a sudden source of interference like a CB radio. Attenuation should increase to account for the increased noise. The properties of the passive step attenuator, however, are fixed upon manufacture.